Many known pearlescent or nacreous pigments are based on micaeous or other lamellar substrates that have been coated with a metal oxide layer. As a result of reflection and refraction of light, these pigments exhibit a pearl-like luster. Depending on the thickness of the metal oxide layer, they can also exhibit interference color effects. A good description of this type of pigment can be found in U.S. Pat. Nos. 3,087,828 and 3,087,829.
The pearlescent pigments most frequently encountered on a commercial basis are titanium dioxide-coated mica and iron oxide-coated mica pearlescent pigments. It is also well-known that the metal oxide layer may be over-coated. For instance, said U.S. Pat. No. 3,087,828 describes the depositing of Fe2 O3 onto a TiO2 layer while U.S. Pat. No. 3,711,308 describes a pigment in which there is a mixed layer of titanium and iron oxides on the mica that is overcoated with titanium dioxide and/or zirconium dioxide.
The oxide coating is in the form of a thin film deposited on the surfaces of the mica particle. The resulting pigment has the optical properties of thin films and thus the color reflected by the pigment arises from light interference which is dependent on the thickness of the coating. Since iron oxide has an inherent red color, a mica coated with this oxide has both a reflection color and an absorption color, the former from interference, the latter from absorption of light. The reflection colors range from yellow to red and the pigments are generally referred to as “bronze”, “copper”, “russet”, etc. The pigments are used for many purposes such as incorporation in plastics and cosmetics as well as outdoor applications such as automotive paints.
Pearlescent pigments containing ferrites are also known. For example, U.S. Pat. No. 5,344,488 and DE 4120747 describe the deposition of zinc oxide onto mica platelets which had been coated with iron oxide. The U.S. patent states that in order to avoid the disadvantage of conventional zinc oxide/mica pigments, namely the tendency to agglomerate, and to obtain a pigment which had good skin compatibility, anti-bacterial action, favorable optical absorption properties and a surface color, the zinc oxide layer is applied to a previously prepared metal oxide-coated plate-like substrate. When calcined, small needle shaped crystallites are randomly distributed on the surface layer so that the zinc ferrite layer obtained is not entirely continuous. The patent states that unlike substrates covered entirely with zinc oxide in a continuous layer, the substrates covered with a layer containing crystallites show only a slight tendency to agglomerate.
The quality of a pearlescent pigment is generally dependent upon the smoothness or continuousness of the coating on the micaceous substrate. The quality of the pigment decreases rapidly with increasing discontinuities in the coating. Said U.S. Pat. No. 5,344,488 indicates that discontinuity is essential in order to avoid agglomeration. Therefore, quality must be sacrificed in order to obtain a usable pigment.
Nitta, U.S. Pat. No. 4,828,623, discloses a process of increasing the water resistance of a titanium dioxide-coated mica pearlescent pigment, which may or may not be aluminum treated, by coating the base material with a hydrated zirconium oxide formed in the presence of hypophosphite. This pigment may be thereafter treated with a silane coupling agent. However, Nitta points out in a later patent, U.S. Pat. No. 5,223,034, that the pigment so made does not stand up under outdoor weather exposure tests and accelerated weatherability tests. The later patent, therefore, overcoats the earlier product with a hydrated cobalt, magnesium or cerium oxide.
Frequently metal oxide coated substrates are calcined and then further processed in water to add another coating. One example is commonly assigned U.S. Pat. No. 5,759,255 disclosing pearlescent pigment for exterior use where improved humidity resistance and weatherability of a metal oxide-coated mica is achieved by treatment with an aluminum or an aluminum-cerium combined with a hydrolyzed silane coupling agent. These overcoats are applied by dispersing metal oxide coated mica substrates in an aqueous system.
We have discovered that calcined ferrite or iron oxide coated natural mica as disclosed in our U.S. Pat. No. 6,139,615 is not stable in aqueous systems and as a result, the natural mica delaminates and/or the ferrite or iron oxide coating separates from the natural mica during the coating process.
FIGS. 4-5 show iron oxide coated natural mica that was calcined at 850° C. for 20 minutes without any outer layer treatment. FIG. 4 is at 20,000× magnification and the sample shows cracks and the iron oxide coating peeling off of the natural mica. FIG. 5 is at 60,000× magnification and the sample shows a bumpy iron oxide coating.
Topy U.S. Pat. No. 5,741,355 teaches that iron oxide may be coated onto synthetic mica. Sun Chemical's SunShine products comprise synthetic fluorophlogopite coated with TiO2 and optionally Fe2O3, carmine, or iron blue. Most grades are available with particle size distributions of 9-45, 20-95, 40-250, and 95-730 microns. Sun Chemical's SunShine Crystal product comprises synthetic mica coated with TiO2 and/or Fe2O3. These pigments have a particle size distribution of 25-150 microns. Sun Chemical US Patent Application Publication 2005/0142084 discloses cosmetics containing nacreous pigments made of coated synthetic mica wherein the particle size is 150-1,000 microns.
DuPont U.S. Pat. No. 3,087,828 has an example wherein a synthetic phlogopite substrate is coated with titanium dioxide. No calcination step is mentioned.
Li Teiqing et al., “Preparation of Synthetic Mica Pearly Luster Pigment”, Journal of Synthetic Crystals, Volume 19, No. 2, pages 166-171 (May 1990) teaches fluorophlogopite coated with titanium dioxide.
It is desired to provide a pigment having improved water stability prior to treatment with an outer layer for providing improved humidity resistance and weatherability and the like.